Organic light-emitting diode

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An organic light-emitting diode (OLED) is a special type of light-emitting diode (LED) in which the emissive layer comprises a thin-film of certain organic compounds. The emissive electroluminescent layer can include a polymeric substance that allows the deposition of very suitable organic compounds, for example, in rows and columns on a flat carrier by using a simple "printing" method to create a matrix of pixels which can emit different colored light. Such systems can be used in television screens, computer displays, portable system screens, advertising and information, and indication applications etc. OLEDs can also be used in light sources for general space illumination. OLEDs lend themselves for the implementation of large areal light-emitting elements. OLEDs typically emit less light per area than inorganic solid-state based LEDs which are usually designed for use as point light sources. Prior to standardization, OLED technology was also referred to as OEL or Organic Electro-Luminescence.

One of the great benefits of an OLED display over the traditional LCD displays is that OLEDs do not require a backlight to function. This means that they draw far less power and, when powered from a battery, can operate longer on the same charge. It is also known that OLED based display devices can be more effectively manufactured than liquid-crystal and plasma displays. However, degradation of OLED materials (see drawbacks) have limited the use of these materials.

Contents

[edit] History

Bernanose and coworkers first produced electroluminescence in organic materials by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine. [1] In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene. [2] and [1]).

The low electrical conductivity of such materials limited light output until more conductive organic materials became available, especially the polyacetylene, polypyrrole, and polyaniline "Blacks" ( AKA "Melanins" ). In 1963 series of papers, Weiss et al first reported [2] high conductivity in iodine-"doped" oxidized polypyrrole. They achieved a conductivity of 1 S/cm. Unfortunately, this discovery was "lost", as was a 1974 report [3] of a melanin-based bistable switch with a high conductivity "ON" state. This material emited a flash of light when it switched.

In a subsequent 1977 paper, Shirakawa et al. reported high conductivity in similarly oxidized and iodine-doped polyacetylene. These researchers received the 2000 Nobel prize in Chemistry for "The discovery and development of conductive organic polymers". The Nobel citation made no reference to the earlier discoveries.

Modern work with electroluminescence in such polymers culminated with Burroughs et al.'s 1990 paper in the journal Nature reporting a very high efficiency green-light-emiting polymer.

[edit] Related technologies

[edit] Small molecules and polymers

The largest OLED display prototype as of May 2005, at 40 inches.
Enlarge
The largest OLED display prototype as of May 2005, at 40 inches.

Small-molecule OLED technology was developed by Eastman-Kodak. The production of small-molecule displays requires vacuum deposition which makes the production process more expensive than other processing techniques (see below). Since this is typically carried out on glass substrates, these displays are also not flexible, though this limitation is not inherent to small molecule organic materials. The term OLED traditionally refers to this type of device, though some are using the term SM-OLED.

A second technology, developed by Cambridge Display Technologies or CDT, is called LEP or Light-Emitting Polymer, though these devices are better known as polymer light-emitting diodes (PLEDs). No vacuum is required, and the emissive materials can be applied on the substrate by a technique derived from commercial inkjet printing. This means that PLED displays can be made in a very flexible and inexpensive way.

Recently a third hybrid light-emitting layer has been developed that uses nonconductive polymers doped with light-emitting, conductive molecules. The polymer is used for its production and mechanical advantages without worrying about optical properties. The small molecules then emit the light and have the same longevity that they have in the SM-OLEDs.

[edit] PLED

A polymer light-emitting diode is an electroluminescent polymer that emits light when subjected to an electric current. It is used as a thin film for full-spectrum color displays and requires a relatively small amount of power for the light produced.

[edit] Functionality

An OLED works on the principle of electroluminescence. The key to the operation of an OLED is an organic luminophore. An exciton, which consists of a bound, excited electron and hole pair, is generated inside the emissive layer. When the exciton's electron and hole combine, a photon can be emitted. A major challenge in OLED manufacture is tuning the device such that an equal number of holes and electrons meet in the emissive layer. This is difficult because, in an organic compound, the mobility of an electron is much lower than that of a hole.

An exciton can be in one of two states, singlet or triplet. Only one in four excitons is a singlet. The materials currently employed in the emissive layer are typically fluorophors, which can only emit light when a singlet exciton forms, which reduces the OLED's efficiency.

Luckily, by incorporating transition metals into a small-molecule OLED, the triplet and singlet states can be mixed by spin-orbit coupling, which leads to emission from the triplet state. However, this emission is always redshifted, making blue light more difficult to achieve from a triplet excited state. It is pointed out that triplet emitters can be four times more efficient than OLED technology [3].

To create the excitons, a thin film of the luminophore is sandwiched between electrodes of differing work functions. Electrons are injected into one side from a metal cathode, while holes are injected in the other from an anode. The electron and hole move into the emissive layer and can meet to form an exciton. Mechanisms and details of exciton formation are discussed in [3] and [4].

Derivatives of PPV, poly(p-phenylene vinylene) and poly(fluorene), are commonly used as polymer luminophores in OLEDs. Indium tin oxide is a common transparent anode, while aluminium or calcium are common cathode materials. Other materials[5] are added between the emissive layer and the cathode or the anode to facilitate or hinder hole or electron injection, thereby enhancing the OLED efficiency..

[edit] Advantages

The radically different manufacturing process of OLEDs lends itself to many advantages over flat panel displays made with LCD technology. Since OLEDs can be printed onto any suitable substrate using inkjet printer technology, they can theoretically have a significantly lower cost than LCDs or plasma displays. The fact that OLEDs can be printed onto flexible substrates opens the door to new applications such as roll-up displays or even displays embedded in clothing.

The range of colors, brightness, and viewing angle possible with OLEDs are greater than that of LCDs because OLED pixels directly emit light. Because of this, OLED pixel colors appear correct and unshifted, even as the viewing angle approaches 90 degrees from the axis perpendicular to the display. LCDs employ a backlight and are incapable of showing true black, while an "off" OLED element produces no light and consumes no power. In LCDs, energy is also wasted because a liquid crystal display requires polarizers which filters out about half of the light emitted by the backlight.

[edit] Drawbacks

The biggest technical problem left to overcome has been the limited lifetime of the organic materials. Particularly, blue OLEDs typically have lifetimes of around 5,000 hours when used for flat panel displays, which is lower than typical lifetimes of LCD or Plasma technology. However, recent experimentation has shown that it's possible to swap the chemical component for a phosphorescent one, if the subtle differences in energy transitions are accounted for, resulting in lifetimes of up to 20,000 hours for blue PHOLEDs.

Also, the intrusion of water into displays can damage or destroy the organic materials. Therefore, improved sealing processes are important for practical manufacturing and may limit the longevity of more flexible displays.

Commercial development of the technology is also restrained by patents held by Eastman Kodak and other firms, requiring other companies to acquire a license. In the past, many display technologies have become widespread only once the patents had expired; aperture grille CRT is a classic example.

[edit] Commercial uses

OLED technology is being used in commercial applications such as small screens for mobile phones and portable digital music players (MP3 players), car radios and digital cameras and also in high resolution microdisplays for head-mounted displays. Such portable applications favor the high light output of OLEDs for readability in sunlight, combined with their low power drain. Unlike (e.g.) television or computer displays, such portable displays are also used intermittently, so the somewhat lower lifespan of OLEDs is not important. Also, prototypes have been made of flexible and rollable displays which take advantage of OLEDs unique characteristics. OLEDs have also been found in models of the Sony Walkman and of some of the Sony Ericsson phones, notably the Z610i, as well as most Motorola color cell phones.

OLEDs could also be used as solid state light sources. As by now the OLED efficacies and lifetime already go beyond those of tungsten bulbs, white OLEDs are under worldwide investigation as source for general illumination (e.g. the EU OLLA project[4]).

eMagin Corporation is currently the only manufacturer of active matrix OLED-on-silicon displays. These are currently being developed for the US military soldiers, the medical field and the future of entertainment where an individual can immerse themselves in a movie or a videogame.

[edit] References

  1. ^ [1] A. Bernanose, M. Comte, P. Vouaux, J. Chim. Phys. 1953, 50, 64. [2] A. Bernanose, P. Vouaux, J. Chim. Phys. 1953, 50, 261. [3] A. Bernanose, J. Chim. Phys. 1955, 52, 396. [4] A. Bernanose, P. Vouaux, J. Chim. Phys. 1955, 52, 509.)
  2. ^ E. Gurnee, R. Fernandez, US Patent 3 172 862, 1965. Ref. Bernius et al. Adv. Mater. 2000.
  3. ^ a b Hartmut Yersin, Triplet emitters for OLEDs. Introduction to exciton formation, charge transfer states, and triplet harvesting
  4. ^ H. Yersin, Triplet emitters for OLED applications. Mechanisms of exciton trapping and control of emission properties. Top. Curr. Chem. 241,
  5. ^ OD Software Incorporated - Material Knowledge Base
  • Shinar, Joseph (Ed.), Organic Light-Emitting Devices: A Survey. NY: Springer-Verlag (2004). ISBN 0-387-95343-4.

[edit] See also

[edit] External links

Display Technologies
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Free-space display
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